textiter: bug fix in forward_search() and backward_search()

textiter: bug fix in forward_search() and backward_search()

When the search is case sensitive, g_utf8_normalize() is not called, so
the skip_decomp argument of the function forward_chars_with_skipping()
must be FALSE.

To verify that, when searching "éb", the count parameter of
forward_chars_with_skipping() have a different value:
- case sensitive: count = 2
- case insensitive: count = 3 (g_utf8_normalize() has been called)

The commit adds unit tests that didn't pass before, and that now pass
(without known regression, obviously).

https://bugzilla.gnome.org/show_bug.cgi?id=702977